Prior art FIG. 1 illustrates a conventional LED 10 flip chip mounted on a portion of a submount wafer 22. In a flip-chip, both the n and p contacts are formed on the same side of the LED die opposite to the growth substrate 12 side.
In FIG. 1, the LED 10 is formed of semiconductor epitaxial layers, including an n-layer, an active layer, and a p-layer, grown on a growth substrate 12, such as a sapphire substrate. In one example, the epitaxial layers are GaN based, and the active layer emits blue light. Any other type of flip chip LED is applicable to the present invention.
Metal electrodes 14 are formed on the LED 10 that electrically contact the p-layer, and metal electrodes 16 are formed on the LED 10 that electrically contact the n-layer. In one example, the electrodes are gold bumps that are ultrasonically welded to anode and cathode metal pads 18 and 20 on a ceramic submount wafer 22. The submount wafer 22 has conductive vias 24 leading to bottom metal pads 26 and 28 for bonding to a printed circuit board. Many LEDs are mounted on the submount wafer 22 and will be later singulated to form individual LEDs/submounts.
Further details of LEDs can be found in the assignee's U.S. Pat. Nos. 6,649,440 and 6,274,399, and U.S. Patent Publications US 2006/0281203 A1 and 2005/0269582 A1, all incorporated herein by reference.
An underfill material 30 is then injected under and around the LED 10 to fill in air gaps between the LED 10 and submount wafer 22. The underfill material 30 is typically liquid epoxy that is then cured to harden. The hardened underfill provides structural support and protects the chip from contaminants. The underfill material 30 is injected by a nozzle 32 that is moved around the LED 10 while injecting the underfill material 30 at a relatively high pressure to fill the narrow gap between the LED 10 and submount wafer 22. The underfill may extend further laterally than shown in the figures in actual devices.
Any excess underfill material 30 (e.g., epoxy) on top of and around the LED 10/substrate 12 may be removed by microbead blasting.
After the underfill material 30 is cured and microbead blasted, the growth substrate 12 is then removed using a laser lift-off process (not shown). The photon energy of the laser (e.g., an excimer laser) is selected to be above the band gap of the LED material and below the absorption edge of the sapphire substrate (e.g., between 3.44 eV and 6 eV). Pulses from the laser through the sapphire are converted to thermal energy within the first 100 nm of the LED material. The generated temperature is in excess of 1000° C. and dissociates the gallium and nitrogen. The resulting high gas pressure pushes the substrate away from the epitaxial layers to release the substrate from the layers, and the loose substrate is then simply removed from the LED structure. The underfill helps prevent the thin LED layers from cracking under the high pressure.
The growth substrate 12 may instead be removed by etching, such as reactive ion etching (RIE), or grinding. Other techniques may be used depending on the type of LED and substrate. In one example, the substrate is Si-based and an insulating material between the substrate and the LED layers is etched away by a wet etch technique to remove the substrate.
After any other wafer-level processes, the submount wafer 22 is then sawed or scribed and broken to singulate the LEDs/submounts. The submounts may then be soldered to a printed circuit board.
Problems with the prior art underfill technique include the following.
Providing a precise amount of underfill material to only fill under and around the thin LED layers is difficult and time-consuming. The underfill process is sequentially performed on an array of LEDs mounted on a submount wafer, prior to the LEDs being singulated. There may be 500-4000 LEDs mounted on a single submount wafer, depending on the size of each LED and the density. Injecting the underfill material under each LED in the array using a single moving nozzle may take 10-40 minutes, depending on the number of LEDs.
Another problem is that the properties of the underfill material must be carefully selected for proper viscosity, thermal expansion, reliability over the long lifetime of the LED, dielectric properties, thermal conductivity, contaminant protection, and other factors. If the viscosity is too high, the pressure needed to inject the underfill under the LED to fill all voids may damage the LED. Voids must be eliminated since any air will expand when the LED/submount becomes hot and push the LED off the submount. Further, since a void area does not support the LED during the laser lift-off process, the downward stress on the LED during the laser lift-off process may crack the LED.
Thermal expansion of the underfill is extremely important since the LEDs undergo a solder reflow process when soldering a singulated LED/submount to a printed circuit board. Such temperatures may be 265° C. The solder reflow temperature is above the typical 185° C. glass transition temperature for epoxy, which is the typical underfill material. As it relates to an epoxy, the glass transition temperature (Tg) is the temperature at which epoxy becomes soft. Above the glass transition temperature, the epoxy thermal expansion rises significantly, causing upward pressure on the LED, resulting in cracking or lifting off of the LED.
What is needed is an improved technique for underfilling an LED that avoids the above-mentioned problems and material limitations.